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Narrative:The homebuilt airplane (TT 114.3 hours & 22.4 hrs since last condition inspection) was destroyed following an in-flight separation of the left wing in level flight and the subsequent uncontrolled flight into terrain. The two-place (tandem) seating, open cockpit, low wing, fabric covered airplane incorporated a tubular steel welded fuselage and empennage, with wooden wings. The left wing separation points were at the wing root where the wooden main wing spar and rear wing spar passed through the tubular brackets of the fuselage. Examination of the failed surfaces revealed that the left wing separated from the aircraft in a predominately upward direction. The main spar was the first to separate in an upward direction. The rear spar then separated in an upward direction with a slight movement towards the rear. The probable cause of the separation was the choice of basswood for the spars instead of the specified spruce. According to the designer, aircraft grade spruce is the only wood specified for the spars. Builder records indicated the use of basswood for spar construction. Examination confirmed the wood spars were basswood. Basswood is considerably weaker and less stiff than spruce. The FAA Advisory Circular 43.13-1B/2A, Chapter 1, Wood Structures, Table 1-1. Selection and Properties of Aircraft Wood, indicates that spruce is excellent for all uses and is considered as the standard by the FAA. Table 1-1 does not list basswood as a selection of aircraft wood. Chapter 1 Paragraph 1-41 states in part: The type of wood used is critical to the structural strength of the aircraft. CAUSE: The in-flight separation of the left wing due to the failure of the wing spars resulting from the builder's improper design change utilizing basswood for the spars instead of the specified spruce.